Lithography systems using deep ultraviolet (UV) illumination at a 193 nm wavelength and corresponding metrology systems are well known, and components for use in such systems have been developed to a significant degree.
A next step in lithography is the use of Extreme Ultraviolet (EUV) illumination in a band about a wavelength of 13.5 nm. Components and corresponding metrology and inspection systems (e.g., actinic reticle inspection systems) for use with EUV light have been developed to a lesser degree.
In one method, to produce EUV light for use in EUV systems, a Nd:YAG laser at a wavelength near 1030 nm operates as a driver laser and projects laser light onto a light source comprising a Xenon target which is stimulated to output the EUV light. In such systems, 1030 nm driver laser light may enter the system or light output from the Xenon target may include light outside of the band desired for illumination of the reticle. Such undesired light transmission may cause thermal damage to a reticle, may cause image flare in a detector image or may have other deleterious effects. Spectral purity filters (SPFs) of various configurations have been proposed to reduce or eliminate unwanted wavelengths of light. U.S. Pat. No. 7,453,645 describes a device including one or more apertures through which light to be filtered is passed. The aperture diameter is selected such that diffraction properties selectively transmit or reflect light of desired wavelengths. United States Patent Publication 2006/0221440 describes a multi-layered, interference, spectral filter in a honeycomb array for use in a lithography system.
While metrology and inspection systems such as actinic (at-wavelength) reticle inspection systems are known for use with conventional UV systems, such systems are still being developed for use with EUV light. FIG. 1 is a schematic illustration of a conventional inspection system 100 for use with UV light. Inspection system 100 includes a light source 110 to produce the UV light, an illumination system 120 to project light onto reticle 130 (also referred to herein as a mask), a projection system 140 to produce an image of the reticle, and a detector array 150 to receive the image of the reticle for subsequent analysis. Such systems typically include a beam splitter 160 to produce a reference beam 162 to measure stability and uniformity of the UV light beam over time and at different locations within the beam using a light measurement apparatus 170. Such beam splitters are made from conventional beam splitter materials including fused silica, magnesium fluoride or calcium fluoride. Such materials are inappropriate for use with EUV light due to their low transmission rates at EUV wavelengths.